Control principle of a moving-coil magnetic-levitation planar motor is basically similar to the control principle of a common linear motor, i.e., adjusting an angle of a current flowing through the motor's three-phase coils based on the position of the coils in a magnetic field such that the motor exerts a constant force in a desired direction. Such control is vector control based on ID and IQ decoupling. Therefore, whether an accurate initial angle of magnetic alignment is obtained not only determines whether generation of maximum horizontal and vertical forces can be ensured for the moving-coil maglev planar motor but also affects the motor's decoupling between the horizontal and vertical directions. If the angle of magnetic alignment is inaccurate or the decoupling between the horizontal and vertical directions is not perfect, crosstalk between the horizontal and vertical directions will be introduced in the control, which will decrease servo performance of the motor.
U.S. Pat. No. 7,205,741B2 discloses an existing system for determining an initial angle of magnetic alignment. As shown in FIG. 1, this solution includes: positioning a sensitive component 3 formed of a layer of a compressible material between coils 1 and a magnet 2 of a motor; using a capacitive or inductive distance sensor 4 to determine a compression of the sensitive component 3 occurring upon the motor exerting a vertical force; searching for the initial angle of magnetic alignment of the motor within a range for magnetic alignment by varying an angle of a current supplied to the three-phase coils and determining a corresponding deformation of the sensitive component 3.
The accuracy of the above-mentioned system for determining angle of magnetic alignment depends both on the deformation of the sensitive component 3 occurring upon the motor generating the vertical force and on the resolution of the distance sensor 4. This requires a good linearity between the generated vertical force and the deformation of the sensitive component 3. However, when the vertical force is too large or too small, the linearity will become insufficient to ensure a satisfying accuracy of the obtained magnetic alignment angle.